Multiple-color air jet looms have a plurality of different kinds of weft threads in readiness for weft insertion and successively insert them, one at a time, through warp sheds according to a fabric to be woven on the loom. Such different weft threads typically vary in thickness. They are subjected to slightly different resistances to being drawn for weft insertion and are slightly differently carried on jets of air.
In prior multiple-color air jet looms, however, air under uniform pressure is ejected from groups of main nozzles and subnozzles irrespectively of the kind of selected weft thread to be inserted. This means that the weft carrying force remains constant at all times for different weft threads, and that the speed at which the weft threads are taken through the warp sheds varies from one weft type to another. For alternately inserting thin and thick weft threads, for instance, the pressure of air jets is preselected and fixed to meet the thick weft thread which requires a greater weft carrying force. Therefore, a certain amount of air under pressure is wasted when inserting the thinner weft threads.